Hi folks,
Just been to Maidenhed Aquatics and bought some fish/assassin snails. When we got home one of the 5 harlequins was floating on the top, upside down but still alive.
I put it in some tank water (removed some from the tank to allow for the water in the bag) with some tonic and swimbladder stuff. Not sure if the kindest thing would be to put it out of its misery. I can't take it back now as the shop closed shortly after we left and I won't be able to take it back as I won't have time after work. Our lass feels sad about it and would like to know what to do.
Thanks in advance..

If this occurred due to an internal injury, as can happen if the fish was hit with the frame of the net or pinned against the glass or bag, they are unlikely to recover. Similar symptoms can also appear solely from stress, and in this case the fish would usually recover, though sometimes not. A third explanation again for identical symptoms is too sudden change in water parameters (hardness, pH and temp) and here too recovery is yes and no.

Since the others are fine and this one died within a few hours, I would ask for a replacement. The store may want the dead fish brought in, policy varies.